The present invention relates to a high-speed video camera capable of taking images of high speed phenomena, storing data of the images on a memory and reproducing the images based on the image data stored on the memory.
If a conventional video camera is used for taking video of a high-speed phenomenon that may unexpectedly occur and does not allow the user to preset optimal photographing conditions, it cannot capture the subject in time with its automatic diaphragm, resulting in viewing fuzzy video with considerable afterglow of pixels therein on a display screen.
In recent years, there has been developed an image sensor comprising a matrix of MOS type light sensor circuits each representing a unit pixel, which has a wide dynamic range suitable for capturing high speed phenomena to visualize with least occurrence of afterglow of pixels.
Japanese Laid-Open Patent Publication No. 2000-329616 discloses an image sensor using a number of light sensor circuits each representing a unit pixel, which circuit comprises, as shown in FIG. 1, a photo-diode PD operating as a photoelectric converting element for producing a sensor current proportional to the quantity of incident light Ls falling thereon, a transistor Q1 having a logarithmic output characteristic in a weak inverse state for converting the sensor current produced in the photodiode into a voltage signal Vpd, a transistor Q2 for amplifying the voltage signal Vpd and a transistor Q3 for outputting a sensor signal in accordance with a timing pulse Vs and which circuit is characterized by its wide dynamic range, thereby achieving the high sensitivity of detecting a light signal. In addition, the light sensor circuit is provided with a means for changing a drain voltage VD of the transistor Q1 to a value lower than a normal value for a specified period to remove a charge accumulated in a parasitic capacitor C of the photodiode PD to initialize the circuit. The light sensor circuit can thus obtain a voltage signal Vpd corresponding to the quantity of incident light Ls even if the sensor current absurdly changed, thereby eliminating the possibility of occurrence of afterglow of the pixel even at a small quantity of incident light.
However, the conventional image sensor using the above-described light sensor circuits cannot be free from structure-derived variations in output characteristics as well as variations in temperature characteristics of pixel signals. In other words, both kinds of the variations shall be compensated for, otherwise, dark-current and bright-current image data cannot be obtained when taking images. The compensation of the image sensor for variations in its output characteristics may be carried out by performing operations referring to a table containing compensating values predetermined based on the measured variations of every sensor circuit. In the case when the conventional image sensor is used for taking an image from a high speed phenomenon by temporally storing obtained image data on a high speed memory and displaying the data on a display screen, it is needed to promptly compensate the image data for variations in output characteristics of respective pixels before storing the image data on the high speed memory. However, it is very difficult to provide the image sensor with a compensating circuit capable of compensating pixel outputs of the image sensor in time at a speed high enough to follow the high speed phenomenon.
Thus, when a high speed subject is captured, fast stored and displayed by the conventional image sensor using a matrix of MOS type light sensor circuits (pixels) having a wide dynamic range with least occurrence of afterglow of pixels, there still arises a problem that it is almost impossible to compensate the image sensor for variations in output characteristics of respective pixels before storing data of the captured image on a memory by following the high speed phenomenon.
Furthermore, when displaying an image taken from the high speed phenomenon being captured by the image sensor in order to monitor the state of taking video thereof, there may be such a problem that the frame rate of the image sensor exceeds the reproducing speed of the display device which in this case cannot display the image.